Heres a few articles to read. Hopefully they will help you.
IMO I would QT the fish and use the Hyposalinity option.
If you can't QT the fish try to keep the Water Quality of the tank as good as possible. Many people swear by adding garlic to their food as well. Ive never had ICH myself but I can try to get Lee to jump in on this thread as she is the resident fish doctor.
Marine Ich-Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part I by Steven Pro
Marine Ich-Cryptocaryon irritans - A Discussion of this Parasite and the Treatment Options Available, Part II by Steven Pro